Radeon HD6970 - Suddenly Fan works at 100% all the time, even after restart and without any workload

I have a Radeon HD670 (2GB, GDDR5) and my operating system is Win7 64bit. Everything worked pretty fine all the time, but suddenly the fan increased his Speed to 100% and did not go down. I was just surfing in the internet, so there was no workload for the GPU. Even after a restart and a reinstallation of all drivers the fan constantly works at 100%.